This invention relates in general to splices between electrical cables, and, more specifically to a splice and method of splicing for use with superconducting cables.
Electrical cables, being finite in length, often need to be spliced together to form a very long, electrically continuous, cable. With the usual materials, such as copper or aluminum, used at ambient temperatures, the two cables are generally simply mechanically fastened together or soldered together. Typical of the prior art techniques for joining two conductors together are the crimp rings disclosed by Bennett in U.S. Pat. No. 3,231,964, and soldering as described by McIntosh et al in U.S. Pat. No. 3,517,150.
Superconducting cables, however, have physical characteristics making such simple splicing techniques either unworkable or difficult. Superconductors are materials, often metals or ceramics, that lose all resistance when cooled below a critical temperature. Many materials have superconducting capabilities, although most only superconduct at temperatures approaching 0.degree. K. The most practical for use in superconducting magnets and the like are those that superconduct at or near the boiling temperature of liquid helium; typically, Nb-Zr and Nb-Ti alloys and the compound Nb.sub.3 Sn. The most common method of splicing such superconductors has been the lap splice, where the cable ends are overlapped and soldered together. Such soldered splices exhibit relatively high resistance which can lead to excessive local heating, to the point where the spliced superconductors are raised above the critical superconducting temperature and cease to superconduct.
A number of different methods have been developed in order to connect ends of superconductor cables without interposing a high resistance material, such as solder, between them. Where the cable has multiple strands, simply overlapping the strands of each cable and crimping them together has been proposed by Wada et al. in U.S. Pat. No. 4,794,688. However, this is not effective with bar or other single strand superconductor cables and provides only a mechanical joint which may have insufficient strength for some applications and leaves the superconductor subject to corrosion.
Multi-filament cable ends have been joined by intertwining the superconductor filaments, heating to a diffusion temperature then crimping a sleeve over the connection as described by Smathers in U.S. Pat. No. 5,111,574. This is a complex process which may degrade the superconducting properties and would be difficult to consistently accomplish outside of a laboratory environment.
Jones, in U.S. Pat. No. 4,631,808 places two cable ends in parallel, crimps a sleeve of superconducting material over the ends, then embeds the entire assembly in a conductor. This method, however, is not suitable for a continuous cable to be wound into a coil or the like.
Thus, there is a continuing need for a simple but effective method of splicing ends of superconducting cables together to form a longer cable suitable for winding into magnet coils and the like, where high resistance splices, with the attendant undesirable heating, are avoided and where the splice can be easily made in a factory environment.